This invention relates to blood pumps that have a magnetically suspended rotor and methods of making the same. This invention also includes blood pumps that have a housing, a rotor, a stator member attached to the housing, a means for levitating the rotor such that the rotor is substantially centered within the housing, and means for rotating the rotor. This invention further includes blood pumps that have a geometric configuration that minimizes shear stresses on the blood, enhances biocompatibility and prevents activation of blood platelets, and thereby prevents thrombosis.
The use of rotary pump ventricular assist devices for aiding a patient""s heart in pumping blood is well known. Such rotary pump ventricular assist devices may be connected to a patient""s heart in a left-ventricular assist configuration, in a right-ventricular assist configuration or in a bi-ventricular assist configuration. For instance, if the left-ventricular assist configuration is adopted, the rotary pump is connected between the left ventricle of the patient""s heart and the aorta. Generally, a rotary pump includes a housing having an inlet and an outlet, an impeller positioned within the housing and impeller blades extending from the impeller. The blood enters the inlet of the housing and is pumped by the rotating impeller through the housing to the outlet and into the patient""s circulatory system.
Blood pumps are a unique class of devices. This is so, inter alia, because artificially pumping blood presents many issues that are not present when pumping fluids in pumps that need not be biocompatible. When pumping blood, it is imperative to prevent damage to the blood cells because this can lead to the activation of platelets, coagulation and potentially fatal thrombosis. For instance, because coagulation can result from increased temperatures, the temperature of the blood must be carefully controlled. Moreover, blood cells may coagulate or albumin of the blood denature if the blood temperature reaches forty-two degrees centigrade (42xc2x0 C.). Even at lower temperatures, some adverse effects may occur. If a blood pump is relatively inefficient, the pump can impart excessive energy to the blood, which usually takes the form of heat. Therefore, it is imperative that blood pumps be efficient to avoid transferring heat to the blood. Per force, effective heat management is very important. Further, sudden flow retardations may cause excessive shear stresses.
Moreover, numerous studies have proven that exposing blood to high stresses, such as shear stresses, results in immediate or delayed destruction of blood cells. As a result of the rotation of an impeller, regions of turbulence, jet formation, cavitation and rapid acceleration may be created in blood pumping operations, causing the blood cells flowing through the pump to break down and rupture. Further, edges or protruding surfaces within a blood pump can cause shear stresses and the breakdown of blood cells. Also, the geometric configuration of a pump may cause localized regions of retarded flow or stagnation. Flow stagnation can cause blood elements to deposit on the pump structure, coagulate and possibly result in thrombosis.
Many attempts have been made to meet the design constraints for using blood pumps as ventricular assist devices. One type of conventional rotary pump utilizes mechanical bearings that necessitate a lubricant flush or purge with an external lubricant reservoir for lubricating the bearing and minimizing heat generation. Examples of this type of rotary pump are illustrated in U.S. Pat. Nos. 4,944,722 and 4,846,152 issued to Carriker et al. and Wampler et al., respectively. There are many disadvantages of this type of rotary pump. The percutaneous supply of the lubricant purge fluid degrades the patient""s quality of life and provides the potential for adverse reaction and infection. Seals for the external lubricant are notoriously susceptible to wear and to fluid attack, which may result in leakage. This may cause the pump to seize. Also, an additional pump is needed for delivery of the lubricant to the bearing. Yet another disadvantage of this type of rotary pump is that the bearings need to be replaced over time because of wear due to the bearings directly contacting other pump structures.
In order to eliminate the need for an external purge of lubricant, rotary pumps having a magnetically suspended impeller have been created. By utilizing a magnetically suspended impeller, direct contact between the bearing and other pump structures, as well as external lubricant purges are eliminated. Examples of this type of rotary pump are disclosed in U.S. Pat. Nos. 5,326,344 and 4,688,998 issued to Bramm et al. and Olsen et al. respectively. These types of rotary pumps generally include an impeller positioned within a housing, wherein the impeller is supported and stabilized within the housing by a combination of permanent magnets positioned in the impeller and the housing and electromagnets positioned within the housing. The impeller is rotated by a ferromagnetic stator ring mounted within the housing and electromagnet coils wound around two diametrically opposed projections. The ferromagnetic impeller and the electromagnetic coils are symmetrically positioned with respect to the axis of the rotary pump and thus, impose an axially symmetric force on the fluid passing through a single annular gap formed between the housing and the impeller.
A disadvantage of these types of rotary pumps is that there is only one annular gap for the blood to pass through, which serves competing purposes with respect to fluid flow and the magnetic suspension and rotation of the impeller. Regarding fluid flow, the gap is desired to be large for efficient pumping whereas, for efficient suspension and rotation of the impeller, the gap is desired to be small. In this type of rotary pump, the fluid gap must be relatively small, so as to provide the proper magnetic suspension. This does not allow for efficient pumping of blood because the gap must be made relatively small.
Blood pumps have been designed with hydrodynamic bearings, as opposed to magnetic bearings. Due to the differential pressure across these types of bearings any flushing of these types of bearings is generally minimal. Thus, these types of pumps generally have a relatively stagnant region of blood within the bearing. Therefore, a drawback of these types of pumps is that the blood is relatively stagnant in the regions around the bearings, which can lead to the deposition of blood elements, coagulation and potentially thrombosis.
Another concern with ensuring the biocompatibility of blood pumps is to minimize the size of the blood pumps. By minimizing the size of blood pumps, the amount of foreign surface area that the blood must contact decreases. This decreases the likelihood that the blood will become contaminated or the blood cells will be damaged. There is a competing concern with minimizing the size of blood pumps. As the size of blood pumps decreases, the flow paths become narrower, the required rotational speed becomes higher and the likelihood of increased shear stresses increases. Therefore, it is important in designing relatively small blood pumps to prevent excessive shear stresses.
The blood pumps of the present invention provide for improvements in pumping blood. These features are related, inter alia, to the magnetic suspension of the rotor and enhancing the biocompatibility and reliability of the pumps through certain geometric features of the pumps while simultaneously minimizing the size of the pumps.
The blood pumps of this invention have a housing, a stator or stationary member and a rotor magnetically suspended between the stator and the housing. A primary flow path may be defined between the housing and the rotor and a secondary flow path may be defined between the stator and the rotor. The primary flow path provides the flow path for the substantial majority of blood flowing through the pump. Defined within the pump may be a radial magnetic bearing that suspends the rotor radially within the pump. Preferably, the radial magnetic bearing includes a stack of magnets disposed in the rotor and a stack of magnets disposed within the stator and aligned with those in the rotor. These two stacks of magnets interface across the secondary flow path to suspend the rotor radially from the stator. By providing a secondary flow path across which the radial bearing operates, the primary flow path can be large enough to provide an adequate flow rate or volume of blood pumped.
The pumps may also have a thrust bearing for controlling the axial position of the rotor. In a preferred embodiment, the thrust bearing includes the radial bearing, thrust coils disposed within the housing, pole pieces, disposed within the rotor, a sensor and a controller. The radial bearing also produces a force in the axial direction. The thrust coils which are preferably disposed in the housing, cooperate with the pole pieces, which are preferably disposed in the rotor, to produce a counteracting force in the axial direction. The blood flowing through the pump also imparts an axial force on the rotor. In order to maintain the rotor""s axial position, the pumps preferably have the controller and the sensor referred to above. The sensor preferably includes a sensing coil disposed within the stator that communicates with the rotor to determine the axial position of the rotor. The controller is in electrical communication with the sensor and the thrust coils. Based on the position sensed by the sensing coils, the controller adjusts the electrical current through the thrust coils. This adjusts the axial force exerted on the rotor, so that the rotor can be positioned to its preferred axial location within the pump. In a preferred embodiment the controller is a Virtually Zero Power controller.
The motor stator for driving the rotor is preferably disposed within the housing and communicates with a rotor magnet across the primary flow path to rotate the rotor. By placing the motor stator within the housing, as opposed to the stator or stationary member around which the rotor rotates, the heat generated by the motor can be more easily dissipated away from the blood. This enhances the biocompatibility of the pumps. Further, by placing the motor stator within the housing, larger wire can be used due to the increased volume in the housing relative to the motor stator. This is significant because using larger wire reduces the ohmic heating of the motor stator.
As alluded to above, the rotor is magnetically suspended by the interaction of magnets across the secondary flow path. The secondary flow path is of a size that balances two competing considerations. The secondary flow path must be large enough to provide adequate flushing throughout the secondary flow and thereby prevent stagnation and the collection of blood cells along localized regions of the secondary flow path. Stagnation and the collection of blood cells can lead to thrombosis and eventually embolization. The secondary flow path must also be small enough, so that the radial magnetic bearing operates effectively in order to minimize the size of the pump and the foreign surface area in contact with the blood. Thus, the secondary flow path is optimally sized so as to balance these competing considerations.
In a preferred embodiment, the flow through the secondary flow path can be retrograde, opposing the direction of flow through the primary flow path. Increased flushing of the secondary flow path can be achieved with retrograde flow. It has been found that if flow is in the same direction as the primary flow path, the flow rate is insufficient to provide the desired amount of flushing without further increasing the size of the secondary flow path and the pump.
Preferably, the stator, the rotor and the housing are all shaped so as to create streamlined or relatively smooth flow paths for the blood and to prevent relatively sharp surfaces or edges from contacting the blood. As described above, such surfaces are undesirable because they increase shear stresses on the blood and increase the likelihood that blood cells will collect at such a surface and coagulate.
Other features of the invention are provided below.